PROJECT DESCRIPTION ? This R01 proposal resubmission from The Johns Hopkins University School of Medicine is founded on recent work spearheaded by Dr. Daniel Thorek in close collaboration with basic science and clinical investigators. Based upon extensive preliminary and pilot data, the central hypothesis is that a novel targeted radioimmunotherapy construct, [225Ac]hu11B6, will be an important tool to treat late-stage disseminated and locally advanced prostate cancer. The monocloncal antibody hu11B6 enables highly specific targeting of active-hK2, a protease solely expressed in prostate-derived tissues and prostate carcinoma, that is tightly regulated by the androgen receptor (AR). While treatment is effective if disease is detected early, disseminated prostate cancer is incurable and claims a staggering 30,000 lives/year in the United States. To address our hypothesis that targeted alpha-particle radioimmunotherapy with [225Ac]hu11B6 can effectively ablate late- stage and disseminated disease, we propose three Specific Aims. In Specific Aim 1 (SA1) we establish the global pharmacokinetic profile and survival benefit of the construct in models of 1) osseous metastases and 2) novel genetically engineered models of aggressive adenocarcinoma with appropriate non-specific and excess- blocked controls. Next, in SA2 we profile the microscopic distribution and radiobiological action of [225Ac]hu11B6 in the advanced tumor setting. This rigorous approach enables us to define biological factors which influence uptake, and to evaluate treatment response at the scale upon which alpha-particle therapy exerts its effects. SA3 builds upon our extensive evaluation of the approved alpha-emitting 223Ra in models of advanced disease to use it in combination with [225Ac]hu11B6. This forms an innovative combination alpha- particle therapy strategy to enhance the efficacy of each agent by irradiating both the cancer cells and their surrounding bone metastatic microenvironment, while avoiding overlapping toxicities. The innovation of this proposal derives from the original design of the [225Ac]hu11B6 construct in addition to its evaluation in advanced models that reflect fatal late-stage disease biology. There is a clear biological mandate that AR- amplification is the mechanism by which advanced disease develops to the castrate resistant stage. The proposed approach is distinguished from contemporary targeted prostate cancer radionuclide therapies (such as PSMA- or bombesin-targeting therapies) in that it systematically targets a tumor-associated protein whose expression is exclusively associated with prostate tissue and correlates with AR-activity. These considerations directly influence the overall impact of this proposal, as we respectfully submit that we have developed a uniquely potent and specific tool to eradicate disseminated foci of disease for which there is no current treatment. As indicated by our Preliminary Data, this proposal has the potential to motivate a significant shift in the clinical management of prostate cancer.